The weather seems to be getting wilder and weirder. People are noticing. What are the connections to human-caused climate change? And how can we best communicate what the most recent science is telling us about human-induced and natural changes to weather and climate?

When heavy rains led to devastating floods in the United Kingdom (UK) in January 2014, the then Prime Minister David Cameron stated that he “very much suspects” the floods were linked to climate change. A scientific analysis had concluded that climate change had increased the chances of the rainfall that caused the flooding by an estimated 43% (Schaller et al, 2016). The fact is that warmer air holds more moisture, which generally leads to heavier rainfall. The potential for damage from such extreme events is also increasing, as higher river levels put more properties at risk from flooding; the 2014 UK floods cost US$ 646 million (£451 million) in insurance losses, one of the highest in history (Schaller et al, 2016).

In Australia, the summer of 2013 was the hottest on record. The sustained high temperatures were linked to bushfires in the country’s southeast and severe flooding in its northeast. Conditions were so severe it was dubbed “the angry summer” (Steffen, 2013). According to a scientific analysis, the record heat that summer was made at least five times more likely – a 500% increase in the odds of it occurring – by human-caused warming. This conclusion, using the observed temperature record and climate models, was made with more than 90% confidence (Lewis and Karoly, 2013).

The 2014 UK flooding and 2013 Australian heat wave are just two recent extreme events that scientists have determined were considerably more likely to occur due to human-caused climate change. Such heat waves and heavy downpours are among the classes of extreme events that tend to be more frequent and/or more severe in a warmer world.

But not all extremes are increasing. For example, there has been an overall decrease in the number of very cold days and nights, as would be expected in a warming world. Still, the Intergovernmental Panel on Climate Change (IPCC) in its 2012 report on extremes wrote: “A changing climate leads to changes in the frequency, intensity, spatial extent, duration, and timing of extreme weather and climate events, and can result in unprecedented extreme weather and climate events” (Field et al, 2012). Nonetheless, scientific findings that specific extreme weather and climate events can, in fact, be attributed to human-caused climate change have not been widely reflected in public understanding.

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Another issue for communication is that the response of the climate system to warming includes intensifying the water cycle, leading, for example, to both more droughts and more floods. If the mechanisms by which this occurs – that is higher air temperatures dry out soils, and a warmer atmosphere holds more moisture leading to heavier precipitation – are not explained to non-scientists, the combination of both wetter and drier conditions can seem counter-intuitive.

Furthermore, the causes of specific extremes can be seen as politically charged in some countries where, unfortunately, climate change has become a partisan issue. For example, in the aftermath of an extreme event, such as a fire or flood, some people may see it as insensitive and/or political to discuss human-induced causes of loss of life or property.

[I would say it is immoral not to point out human-induced causes, since that might lead to us dealing with those causes and decreasing the number of such disasters.]

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Even as occurrences of certain classes of extreme events have increased, the media in some countries have not kept pace in communicating the scientific understanding of the connection between climate change and extremes.

For example, in the United States of America (U.S.), an August 2015 study by Media Matters for America (MMA) showed that top newspapers ran coverage of wildfires and of the U.S. Clean Power Plan side-by-side (see image above), but failed to mention the role of human-induced climate change in an unseasonably early wildfire season (MMA, August 2015). While calling the wildfires “the new normal,” major California newspapers neglected to give any explanation of the cause of this new normal (e.g., Westerling et al, 2006). Similarly, in June 2016 MMA noted a reversal of progress in attributing extreme events to climate change when media failed to portray links between climate change and the May-June floods in Texas. They noted that major U.S. broadcast news networks ignored climate change in their coverage of the flooding, marking a deterioration in coverage of the linkages since 2015 when networks covered the science connecting climate change to the May 2015 Texas floods (MMA, June 2016).

When the media does cover climate change impacts, the focus is overwhelmingly on extreme weather events.

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In terms of understanding the linkages between extreme weather and human-induced climate change, the public also tends to be swayed by the views of prominent leaders, even when those views are at odds with the science. For example, an analysis of the record-breaking spring high temperatures that occurred in Australia in 2013 and 2014 showed that the human influence on climate made those record high temperatures substantially more probable (Lewis and Karoly, 2014). Another analysis found that these extreme temperatures were very unlikely to have occurred in the absence of human-caused climate change (Gallant and Lewis, 2016). However, public statements from a prominent leader contradicted these analyses, promoting the view that natural variations and the lengthening period of record could account for the recent heat extremes. Although these views could not be reconciled with the science, they were widely reported and have persisted in public understanding of extreme events.

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In several studies, these three factors have aligned and attribution statements have had a high level of confidence. For example, there is great clarity and confidence in attributing heat events that occur over large areas and extended time periods. The physics are well understood, changes are documented in observations, and they are simulated accurately in climate models. For example, in Australia, 2013 was a year of heat extremes with the hottest day, week, month, summer and year on record. Two separate studies found that the 2013 extreme heat in Australia would have been virtually impossible without human-caused climate change (Knutson et al, 2014; Lewis and Karoly, 2014).

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It is also true that extreme weather events now occur within a climate system where the background conditions have changed. As such, no weather is entirely “natural” anymore, but rather occurs in the context of a changed climate. That is, “Global warming is contributing to an increased incidence of extreme weather because the environment in which all storms form has changed from human activities” (Trenberth, 2011, USA Today). Every event has been influenced by climate change to some extent through increases in heat, atmospheric moisture and sea level, which all influence how extreme events play out (Trenberth et al, 2015).